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Man, it looks like you’re doing a solid job on the whole “knees out” thing. It’s a bit hard to tell in the dim light, but it does look a little “winky” at the bottom. Which just basically goes back to the topic at hand. I think you have the rest in the bag enough to progress this thing.
I wouldn’t necessarily revert completely back to the air squat as a learning tool on this, however. It can be useful, but I think it’s going to be hard to really feel what it’s like to put a stretch on the hamstrings with your bodyweight alone. I would even choose an easy 12-rep weight, and just slowly take it down to your bottom position and hang out for a bit, staying tight and squirming around a bit to feel where that tension’s at.
Something I like to do (unloaded, this time), is just drop down into an air squat and just press my knees back (keeping a braced spine) and feel what it’s like to hit hamstring tension, kind of like when you’re hunting for tension in the bottom position of a deadlift. After I press back and hit tension, I try to “pull” it back with me into the squat bottom position, to be even tighter in the hamstrings. I almost think about that return to the squat position like I’m doing a hamstring curl against the floor. This really helped me figure out my hamstrings in the squat. Even try doing it while holding on to the squat rack, if you find yourself tipping backward a lot.
With regards to using the back squat to learn and improve that bottom position, almost think of it as stretching the hamstrings at the bottom.
Unfortunately, it’s one of those things that’s a lot harder to articulate than I would like. Just keep thinking about it, and a great epiphany will happen sooner or later. And I’d say sooner with the kind of drive you’re putting into improving this position.